what are the things to check when buying a used iphone 7

The main thing is that you can only verify it is not activation locked if you have it in your hands. So if meeting person to person, do not pay until you have seen it boot to the setup and activation menu, and gone far enough in that to ensure you can login to iCloud with your AppleID.

The seller should complete all the steps in What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

However, you can only really verify they have if you can get your hands on the device to check it before paying for it. In the USA you can check if the IMEI is blacklisted currently on any USA carrier at their trade association’s web site - https://stolenphonechecker.org/spc/consumer (this is a service of the USA carrier’s trade association so cannot check other nations or carriers outside the USA).

Absolutely make sure that the phone is not activation locked. Take a look at this. It must be completely erased by the previous owner.

Turn off Find My iPhone Activation Lock - Apple Support

If you are buying it sight unseen, it’s hard to tell you what to look for. If you buy it in person, make sure it turns off an on, inspect the screen and the rest of the phone for any physical damage, and just make sure it works.

I have listed major checks that may be done for your own information and satisfaction

Verify the activation lock

This step ensures that the phone is not a stolen phone. You may check the activation here: www.icloud.com/activationlock/

Warranty Coverage

Check the warranty coverage of the iPhone here: https://checkcoverage.apple.com/us/en/

Check Model Number to know the country of original purchase & also if it is Original / Refurbished / Replacement etc

Understand what the model number of the iPhone here: https://www.theiphonewiki.com/wiki/Models

Insert a SIM and Check for Cellular Service

To ensure the iPhone is not Carrier Locked: https://support.apple.com/en-us/HT201328

SravanKra, that activation lock page has not been available for a couple of years now. Apple shut it down over abuse concerns quite awhile ago. There has been no way to check activation lock since it was shut down, other than physically inspecting the device itself.